An equalizer in general compensates frequency depending losses that a signal experiences when passing through, a transmission channel. Transmission channels include a wire, a pair of wires, an optical fibre, the reading and writing channels of a storage device like a hard-disc. A pair of wires includes a twisted pair, a twinax coax or a differential transmission line on a printed circuit board.
Adaptive equalizers generally include an adaptable filter, a control loop and an output reconstruction unit.
Patent application EP-02447160, co-pending herewith, describes how to organise the control loop such that robust self-adaptation is achieved, independently from the transmit amplitude and the transmitted bit pattern. A feedback control signal is generated from the equalised output of an equalizer filter. Depending on whether the output signal has been under- or over-compensated, the feedback control signal increases or decreases, such that after a reasonable time the feedback control signal converges to a value where optimal compensation is reached. The control loop is formed by a first means for measuring a short-term-amplitude signal of the output signal, a second means for measuring a long-term-amplitude signal of the output signal and a comparator means for comparing the short-term-amplitude signal and the long-term-amplitude signal, and for determining the evolution of the feedback control signal.
U.S. Pat. No. 5,841,810 describes a way to arrange multiple adaptive filter stages in an adaptive filter. The plurality of filter stages have a common equalisation control signal that has a magnitude that corresponds to the communications path transfer function, with each adaptive filter stage transfer function being an approximate inverse of a transfer function that corresponds to a portion of the input data signal communications path. The compensation thus is based on the ideal transfer function of the communication path. Further, a method of making multiple control signals using Zener-diodes is taught. This method has the disadvantage that it cannot be used with a standard CMOS or BICMOS circuit technology.
US patent application US-2002/0034221 discloses a communications receiver that has multiple stages each having a transfer function I+Ki[fi(jφ)], wherein the Ki vary with a sequential gain control methodology. US-2002/0034221 thus teaches compensation by making a sum per stage of the unity input signal linearly added to a function that has higher frequency gain. This known method makes multiple tuning signals in circuitry using many comparators and is relative complex, but is not suited for low voltage operation nor for implementation on a small chip area using small transistors that have large input offset mismatches.
A difficulty with the present state-of-the-art of adaptive equalizers is that it is being taught to make a transfer function of the equalizer filter correspond to an approximate inverse of a transfer function corresponding to a portion of the input data signal communications path. In that way, only very good filter types can be implemented. Similarly, in the state-of-the-art on adaptive filters, as e.g. in U.S. Pat. No. 5,841,810, serially connected cascaded adaptive filter sections or filter stages are used that each generate an approximate inverse of a transfer function of a portion of the input data signal communications path.
In that way, adaptive filters and filter sections require the use of near to perfect tuneable filters. This has a consequence that only a very small number of tuneable filters types can be implemented. These filter types typically further require a power supply voltage that is larger than 2V.